Actin-capping proteins which bind to the barbed ends of actin filaments have been isolated from a variety of non-muscle tissues. We have recently purified such a protein from skeletal muscle. The purpose of this proposal is to take advantage of the unique structure/function relationships of muscle proteins to help define the biological role of this and other actin-capping proteins. We plan to do this by: 1) developing monoclonal and polyclonal antibodies which specifically recognize the capping protein 2) using techniques of indirect immunofluorescence and immunoelectron microscopy to localize the capping protein in muscle and non-muscle tissues 3) identifying and purifying proteins which demonstrate physiological protein-protein interactions with the capping protein and studying their binding to the capping protein in solution and extracted tissues 4) studying the localization of the capping protein during embryogenesis in chick myocardium. Our short term goal will be to explore the possibility that capping proteins may be responsible for mediating the attachment of actin filaments to structures such as the Z-line of skeletal muscle. The long term goal of this proposal is to develop an understanding of how the length, number, and attachments of actin filaments to other structures are regulated.